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The rock inside the planet we live on can melt to form molten rock called magma. This magma is lighter than the rocks around it and so it rises upwards. Where the magma eventually reaches the surface we get an eruption and volcanoes form.

The top part of the Earth is made up of a number of hard pieces called tectonic plates. Magma and volcanoes often form where the plates are pulled apart or pushed together but we also find some volcanoes in the middle of tectonic plates.

Volcanoes have many different shapes and sizes, some look like steep mountains (stratovolcanoes), others look like bumps (shield volcanoes) and some are flat with a hole (a crater or caldera) in the centre that is often filled with water.

The shape of the volcano and how explosively it erupts depend largely on how “sticky” and how “fizzy” (how much gas) the magma is that is erupted.

For example, if you try to blow bubbles in cooking oil though a straw, the bubbles can escape quite easily because the cooking oil is runny.

If you try to blow bubbles in jam or peanut butter you would find it very difficult because the jam and peanut butter are very sticky, they wouldn’t move much at all if you tried to pour them out of the jar.

It is the same with volcanoes. When magma rises towards the surface gas bubbles start to form. Whether or not they can escape as the magma is rising affects how explosive the eruption will be.

Where the magma is runny like cooking oil and doesn’t have much bubbly gas mixed in it, such as places like Hawaii, then we see lots of slow-moving lava flows and shield volcanoes. Lava is what we call magma when it reaches the surface.

Here are some pictures of a recent Hawaiian eruption:

However, where the magma is very sticky, like jam or peanut butter, and if it contains a lot of bubbly gas then the gas can get stuck and eruptions can be very powerful and explosive, like the recent eruptions at Fuego volcano in Guatemala.

Damage caused by eruptions

In explosive eruptions the frothy, bubbly magma can be ripped apart into tiny bits called volcanic ash. This is not ash like you get after a barbecue or fire, it does not crumble away in your fingers. It is very sharp and is dangerous to breathe in.

Some explosive volcanoes can send ash high up into the sky and it can travel around the world over different countries. If aeroplanes travel through an ash cloud from a volcano it can cause a lot of damage to the engine.

Other explosive eruptions create fast-moving, hot clouds of volcanic ash, gas and rocks that travel down the sides of the volcanoes and destroy pretty much everything in their path.

The benefits of volcanoes

Despite the great damage they can cause, volcanoes also help us to live. Volcanic ash provides food for the soil around volcanoes which helps us grow plants to eat. The heat from some volcanoes is used to make energy to power lights, fridges, televisions and computers in people’s houses.

You can find some more information about different types of volcanoes here and here.

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A few false starts

The last major eruption of Mount Agung was in 1963. Since then, there have been two known periods of activity at the volcano site without an ensuing eruption.

In 1989, a few volcanic earthquakes occurred and hot, sulphur-rich gas emissions were observed with no eruption.

Between 2007 to 2009, satellite data showed inflation (swelling) of the volcano at a rate of about 8cm per year, probably caused by the inflow of new magma (molten rock) into the shallow plumbing system. This was followed by deflation for the next two years, again without an eruption.

The current volcanic activity – mainly the number of earthquakes – has not subsided since the alert level was raised to level 4. It continues to fluctuate at high levels, with more than 600 earthquakes a day. This indicates that the threat of an eruption is still high, despite a general decline in overall seismic energy.

This past weekend saw the highest number of daily earthquakes, with more than 1,100 recorded on Saturday October 14.

Graph showing the number of recorded earthquakes per day at Mount Agung volcano. The orange shows shallow volcanic earthquakes, light green is deep volcanic earthquakes and the blue is local tectonic earthquakes.Centre for Volcanology and Geological Hazard Mitigation

The latest statement from the Indonesian Centre for Volcanology and Geological Hazard Mitigation was released on October 5. It said earthquake data indicates that pressure is continuing to build up under the volcano due to the increasing magma volume and as magma moves towards the surface.

It’s all about the gas

Magma contains dissolved gases (volatiles) such as water, carbon dioxide and sulphur dioxide. As magma moves towards the surface, the pressure becomes less and so gas bubbles form, akin to taking the top off a fizzy drink bottle. These gas bubbles take up additional space in the magma and increase the overall pressure of the system.

The amount of gas, and whether or not gas is able to escape from the magma prior to eruption, are major factors that determine how explosive (or not) any volcanic eruption will be.

If the gas bubbles forming in the magma stay within as it ascends beneath Mount Agung, then it could lead to a more explosive eruption. If the gas formed is able to escape, it might depressurise the system enough to erupt less violently or not at all.

White gas plumes, composed mainly of water vapour, have been observed. They have typically reached 50-200m above the crater rim at Mont Agung, and up to 1,500m on October 7. This water vapour is likely due to the hydrologic system heating up in response to the intruding magma at depth.

During the 1963 eruption, Mount Agung produced a significant amount of sulphur-rich gas that caused an estimated global cooling of 0.1-0.4℃. In this current phase of activity, we are yet to see any significant release of sulphur dioxide from the intruding magma.

How big would an eruption be?

It’s not easy to predict how big any eruption at Mount Agung would be. Analysis of volcanic material deposited during previous eruptions over the past 5,000 years suggests that about 25% of them have been of similar or larger size than the 1963 eruption.

The last two eruptions of Mount Agung in 1843 and 1963 had a Volcanic Explosivity Index (VEI) of 5, on a scale of 0-8. A 0 would be something like a lava flow on Hawaii that you could generally walk or run from, and 8 would be a supervolcanic eruption like Yellowstone (640,000 years ago and 2.1 million years ago) in the United States or Toba (74,000 years ago) in North Sumatra, Indonesia.

Based on a history of explosive activity at the volcano, the Indonesian authorities are maintaining the current hazard zone of up to 12km from the summit of Mount Agung.

It’s still considered more likely than not that it will erupt, but the question remains: when?

Antarctica is a vast icy wasteland covered by the world’s largest ice sheet. This ice sheet contains about 90% of fresh water on the planet. It acts as a massive heat sink and its meltwater drives the world’s oceanic circulation. Its existence is therefore a fundamental part of Earth’s climate.

Less well known is that Antarctica is also host to several active volcanoes, part of a huge “volcanic province” which extends for thousands of kilometres along the western edge of the continent. Although the volcanic province has been known and studied for decades, about 100 “new” volcanoes were recently discovered beneath the ice by scientists who used satellite data and ice-penetrating radar to search for hidden peaks.

These sub-ice volcanoes may be dormant. But what would happen if Antarctica’s volcanoes awoke?

We can get some idea by looking to the past. One of Antarctica’s volcanoes, Mount Takahe, is found close to the remote centre of the West Antarctic Ice Sheet. In a new study, scientists implicate Takahe in a series of eruptions rich in ozone-consuming halogens that occurred about 18,000 years ago. These eruptions, they claim, triggered an ancient ozone hole, warmed the southern hemisphere which caused glaciers to melt, and helped bring the last ice age to a close.

Mt Takahe grew over hundreds of thousands of years and its 8km-wide caldera now towers above the ice sheet.NASA / Jim Yungel, CC BY-SA

This sort of environmental impact is unusual. For it to happen again would require a series of eruptions, similarly enriched in halogens, from one or more volcanoes that are currently exposed above the ice. Such a scenario is unlikely although, as the Takahe study shows, not impossible. More likely is that one or more of the many subglacial volcanoes, some of which are known to be active, will erupt at some unknown time in the future.

Eruptions below the ice

Because of the enormous thickness of overlying ice, it is unlikely that volcanic gases would make it into the atmosphere. So an eruption wouldn’t have an impact like that postulated for Takahe. However, the volcanoes would melt huge caverns in the base of the ice and create enormous quantities of meltwater. Because the West Antarctic Ice Sheet is wet rather than frozen to its bed – imagine an ice cube on a kitchen work top – the meltwater would act as a lubricant and could cause the overlying ice to slip and move more rapidly. These volcanoes can also stabilise the ice, however, as they give it something to grip onto – imagine that same ice cube snagging onto a lump-shaped object.

In any case, the volume of water that would be generated by even a large volcano is a pinprick compared with the volume of overlying ice. So a single eruption won’t have much effect on the ice flow. What would make a big difference, is if several volcanoes erupt close to or beneath any of West Antarctica’s prominent “ice streams”.

A velocity map of Antarctic ice streams as they move toward the ocean.NASA/JPL, CC BY-SA

Ice streams are rivers of ice that flow much faster than their surroundings. They are the zones along which most of the ice in Antarctica is delivered to the ocean, and therefore fluctuations in their speed can affect the sea level. If the additional “lubricant” provided by multiple volcanic eruptions was channelled beneath ice streams, the subsequent rapid flow may dump unusual amounts of West Antarctica’s thick interior ice into the ocean, causing sea levels to rise.

Under-ice volcanoes are probably what triggered rapid flow of ancient ice streams into the vast Ross Ice Shelf, Antarctica’s largest ice shelf. Something similar might have occurred about 2,000 years ago with a small volcano in the Hudson Mountains that lie underneath the West Antarctica Ice Sheet – if it erupted again today it could cause the nearby Pine Island Glacier to speed up.

The volcano–ice melt feedback loop

Most dramatically of all, a large series of eruptions could destabilise many more subglacial volcanoes. As volcanoes cool and crystallise, their magma chambers become pressurised and all that prevents the volcanic gases from escaping violently in an eruption is the weight of overlying rock or, in this case, several kilometres of ice. As that ice becomes much thinner, the pressure reduction may trigger eruptions. More eruptions and ice melting would mean even more meltwater being channelled under the ice streams.

Mt Erebus is one of Antarctica’s most active volcanoes. The rocks in the foreground are the remnants of several younger subglacial volcanoes.antarcticglaciers.org, Author provided

Potentially a runaway effect may take place, with the thinning ice triggering more and more eruptions. Something similar occurred in Iceland, which saw an increase in volcanic eruptions when glaciers began to recede at the end of the last ice age.

So it seems the greatest threat from Antarctica’s many volcanoes will be if several erupt within a few decades of each other. If those volcanoes have already grown above the ice and their gases were rich in halogens then enhanced warming and rapid deglaciation may result. But eruptions probably need to take place repeatedly over many tens to hundreds of years to have a climatic impact.

More likely is the generation of large quantities of meltwater during subglacial eruptions that might lubricate West Antarctica’s ice streams. The eruption of even a single volcano situated strategically close to any of Antarctica’s ice streams can cause significant amounts of ice to be swept into the sea. However, the resulting thinning of the inland ice is also likely to trigger further subglacial eruptions generating meltwater over a wider area and potentially causing a runaway effect on ice flow.